Local instruction loop buffer utilizing execution unit register file

ABSTRACT

A method utilizes a register file of an execution unit as a local instruction loop buffer to enable suitable algorithms, such as DSP algorithms, to be fetched and executed directly within the execution unit, and often enabling other logic circuits utilized for other, general purpose workloads to either be powered down or freed up to handle other workloads.

FIELD OF THE INVENTION

The invention is generally related to data processing, and in particularto processor architectures and controlling power consumption of sucharchitectures.

BACKGROUND OF THE INVENTION

As semiconductor technology continues to inch closer to practicallimitations in terms of increases in clock speed, architects areincreasingly focusing on parallelism in processor architectures toobtain performance improvements. At the chip level, multiple processingcores are often disposed on the same chip, functioning in much the samemanner as separate processor chips, or to some extent, as completelyseparate computers. In addition, even within cores, parallelism isemployed through the use of multiple execution units that arespecialized to handle certain types of operations. Pipelining is alsoemployed in many instances so that certain operations that may takemultiple clock cycles to perform are broken up into stages, enablingother operations to be started prior to completion of earlieroperations. Multithreading is also employed to enable multipleinstruction streams to be processed in parallel, enabling more overallwork to performed in any given clock cycle.

These various techniques for improving execution unit performance,however, do not come without a cost. Parallelism adds complexity, oftenrequiring a greater number of logic gates, which increases both the sizeand the power consumption of such execution units. Coupling thesetechniques with the general desire to increase performance through othertechniques, such as increased switching frequency, the power consumptionof complex, high performance execution units continues to increase,despite efforts to reduce such power consumption through processimprovements. Excessive power consumption can present issues forportable or battery powered devices, but more typically, excessive powerconsumption presents issues for nearly all electronic circuits due tothe generation of heat, which often requires elaborate cooling systemsto ensure that a circuit does not overheat and fail.

Chip-wide control over power consumption is often used in electroniccircuits such as those used in laptop computers or other portabledevices, typically by throttling down the clock rate or frequency of thecircuit to reduce power consumption and the generation of heat. Inaddition, power consumption may also be reduced in some instances bytemporarily shutting down unused circuits on a chip, including, forexample, entire execution units. In all of these instances, however,throttling back the power consumption of the circuit usually results inlower performance in the chip. Furthermore, the circuit characteristicsthat define the overall power consumption of such circuits, e.g., cycletime, voltage, logic area, capacitance, etc., are most often designed tomeet a maximum performance target.

Particularly for complex System on Chip (SOC) designs, increasinglycomplex logic circuitry is being incorporated into individual chips, andin many instances, it costs more power per bit to move the bit frommemory to the central processing unit (CPU), than it does to perform thedesired computation. As a result, improved power reduction mechanismsare required for moving data around on, and off, chip. Additionally,many features once unique to digital signal processors (DSPs) areincreasingly being implemented on general purpose processors to reducecost by eliminating the need for separate DSP chips in a system and toincrease performance by eliminating the need to move data between a DSPchip and the CPU.

However, many algorithms more traditionally performed by DSPs, e.g.,Fast Fourier Transforms (FFT), do not perform as well using traditionalgeneral purpose processors or CPUs. Although some features added to morerecent general purpose processor designs, e.g., SIMD execution units andpredication, have significantly improved performance, the powerconsumption of general purpose processing units performing thesealgorithms is still typically much higher than that of DSP chipsspecifically tailored for those algorithms. This is primarily becausegeneral purpose processing units typically incorporate large blocks oflogic such as multiple cache memories, multiple threads of execution,multiple execution units, etc. that are intended to improve performancegenerally for most workloads. However, for many DSP algorithms, thislogic does very little to improve performance, and thus the additionalpower consumption of this logic is often effectively wasted whenexecuting such DSP algorithms in a general purpose processor.

Therefore, a continuing need exists in the art for improved manners ofreducing power consumption in an integrated circuit, particularly inconnection with executing DSP algorithms and the like.

SUMMARY OF THE INVENTION

The invention addresses these and other problems associated with theprior art by providing a method that utilizes a register file of anexecution unit as a local instruction loop buffer to enable suitablealgorithms to be performed directly and efficiently within the executionunit. By utilizing an execution file as a local instruction loop buffer,Instructions may be stored in a register file and fetched, decoded andexecuted directly from the register file, enabling other logic circuitsto either be powered down or freed up to handle other workloads, andthereby reducing power consumption and/or improving overall performanceof a processing unit.

For example, in some embodiments, an auxiliary execution unit such as ascalar or vector floating point execution unit may incorporateadditional logic to implement a local instruction loop buffer in theregister file thereof such that instructions representing a DSPalgorithm can be directly fetched and executed out of the register file,rather than an existing instruction cache. By doing so, logic circuitssuch as other execution units, data caches, data address translationunits, and other hardware threads may be selectively disabled, oralternatively, freed to handle other workloads. As a result, in manyembodiments a general purpose processing unit may be provided withenhanced performance for specialized workloads such as DSP workloadswith reduced power consumption, while also not sacrificing performancefor non-DSP/general purpose workloads.

Therefore, consistent with one aspect of the invention, a method ofexecuting instructions in a processing unit of the type including anexecution unit including a register file includes fetching a firstplurality of instructions from an instruction stream with the processingunit; in response to at least one instruction from the instructionstream, causing the execution unit to manipulate operand data stored inthe register file; in response to at least one instruction from theinstruction stream, loading a second plurality of instructions into theregister file of the execution unit to utilize at least a portion of theregister file as a local instruction loop buffer; fetching the secondplurality of instructions from the register file; and causing theexecution unit to execute the second plurality of instructions fetchedfrom the register file.

These and other advantages and features, which characterize theinvention, are set forth in the claims annexed hereto and forming afurther part hereof. However, for a better understanding of theinvention, and of the advantages and objectives attained through itsuse, reference should be made to the Drawings, and to the accompanyingdescriptive matter, in which there is described exemplary embodiments ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of exemplary automated computing machineryincluding an exemplary computer useful in data processing consistentwith embodiments of the present invention.

FIG. 2 is a block diagram of an exemplary NOC implemented in thecomputer of FIG. 1.

FIG. 3 is a block diagram illustrating in greater detail an exemplaryimplementation of a node from the NOC of FIG. 2.

FIG. 4 is a block diagram illustrating an exemplary implementation of anIP block from the NOC of FIG. 2.

FIG. 5 is a block diagram illustrating an exemplary implementation of anIP block from the NOC of FIG. 2, and configured to implement a localinstruction loop buffer in an execution unit consistent with theinvention.

FIG. 6 is a block diagram of the auxiliary instruction and executionunits referenced in FIG. 5.

FIG. 7 is a flowchart illustrating an example sequence of operationsused to implement a DSP algorithm using the IP block of FIG. 5.

DETAILED DESCRIPTION

Embodiments consistent with the invention utilize a register file of anexecution unit as a local instruction loop buffer to enable suitablealgorithms to be performed directly and efficiently within the executionunit. By doing so, other logic circuits in a processing unit utilizedfor other, general purpose workloads may either be powered down or freedup to handle other workloads, thereby reducing power consumption and/orimproving overall performance of a processing unit. In addition, theneed for a separate, specialized execution unit may be avoided in someembodiments. For example, for DSP applications where DSP workloads maybe required along with other, non-DSP/general purpose workloads, anexecution unit may be configured to perform DSP algorithms using a localinstruction loop buffer, thereby eliminating the need for a separate DSPaccelerator or chip.

A local instruction loop buffer may be implemented, for example, in ageneral purpose processing unit, which within the context of theinvention may include practically any type of processing unit, e.g., asdisposed in an IP block, a processor chip, a processor core, etc., andcapable of executing instructions to handle general purpose workloads.While a general purpose processing unit may include various components,execution units or accelerators that optimize or accelerate particulartypes of workloads, e.g., image processing, graphics, scientificworkloads, transaction processing, etc., a general purpose processingunit is otherwise capable of handling multiple types of workloads, evenif it does so sub-optimally as compared to a more specialized processingunit. In the least, a general purpose processing unit is capable ofhandling workloads other than the particular DSP workloads handled byDSP chips, DSP execution units, or other DSP-specific logic circuits.

A DSP workload relates to the performance of various types of DSPalgorithms, which are characterized as algorithms that manipulatedigital representations of signals, which in turn typically representtime-varying or spatially-varying physical quantities, including, forexample, image processing, audio processing, and processing of sensordata, among other applications.

In addition, utilization of a local instruction loop buffer may enableother logic circuits, e.g., one or more functional units, to be disabledand/or powered down. A functional unit, within the context of theinvention, may include an execution unit such as a fixed point executionunit (XU), a floating point execution unit (FPU), an auxiliary executionunit (AXU), or various types of accelerators or specialized executionunits (e.g., encryption/decryption engines, DMA engines,compression/decompression engines, physics engines, graphics processors,coprocessors, etc.). In addition, a functional unit may include types ofprocessor logic other than execution units or accelerators, e.g.,caches, multithreading logic, pipeline stages, instances or components,memory management logic, address translation logic, interface logic,prediction logic, renaming logic, issue logic, decode logic, completionlogic, etc.

Other variations and modifications will be apparent to one of ordinaryskill in the art. Therefore, the invention is not limited to thespecific implementations discussed herein.

Hardware and Software Environment

Now turning to the drawings, wherein like numbers denote like partsthroughout the several views, FIG. 1 illustrates exemplary automatedcomputing machinery including an exemplary computer 10 useful in dataprocessing consistent with embodiments of the present invention.Computer 10 of FIG. 1 includes at least one computer processor 12 or‘CPU’ as well as random access memory 14 (‘RAM’), which is connectedthrough a high speed memory bus 16 and bus adapter 18 to processor 12and to other components of the computer 10.

Stored in RAM 14 is an application program 20, a module of user-levelcomputer program instructions for carrying out particular dataprocessing tasks such as, for example, word processing, spreadsheets,database operations, video gaming, stock market simulations, atomicquantum process simulations, or other user-level applications. Alsostored in RAM 14 is an operating system 22. Operating systems useful inconnection with embodiments of the invention include UNIX™, Linux™Microsoft Windows XP™, AIX™, IBM's i5/OS™, and others as will occur tothose of skill in the art. Operating system 22 and application 20 in theexample of FIG. 1 are shown in RAM 14, but many components of suchsoftware typically are stored in non-volatile memory also, e.g., on adisk drive 24.

As will become more apparent below, embodiments consistent with theinvention may be implemented within Network On Chip (NOC) integratedcircuit devices, or chips, and as such, computer 10 is illustratedincluding two exemplary NOCs: a video adapter 26 and a coprocessor 28.NOC video adapter 26, which may alternatively be referred to as agraphics adapter, is an example of an I/O adapter specially designed forgraphic output to a display device 30 such as a display screen orcomputer monitor. NOC video adapter 26 is connected to processor 12through a high speed video bus 32, bus adapter 18, and the front sidebus 34, which is also a high speed bus. NOC Coprocessor 28 is connectedto processor 12 through bus adapter 18, and front side buses 34 and 36,which is also a high speed bus. The NOC coprocessor of FIG. 1 may beoptimized, for example, to accelerate particular data processing tasksat the behest of the main processor 12.

The exemplary NOC video adapter 26 and NOC coprocessor 28 of FIG. 1 eachinclude a NOC, including integrated processor (‘IP’) blocks, routers,memory communications controllers, and network interface controllers,the details of which will be discussed in greater detail below inconnection with FIGS. 2-3. The NOC video adapter and NOC coprocessor areeach optimized for programs that use parallel processing and alsorequire fast random access to shared memory. It will be appreciated byone of ordinary skill in the art having the benefit of the instantdisclosure, however, that the invention may be implemented in devicesand device architectures other than NOC devices and devicearchitectures. The invention is therefore not limited to implementationwithin an NOC device.

Computer 10 of FIG. 1 includes disk drive adapter 38 coupled through anexpansion bus 40 and bus adapter 18 to processor 12 and other componentsof the computer 10. Disk drive adapter 38 connects non-volatile datastorage to the computer 10 in the form of disk drive 24, and may beimplemented, for example, using Integrated Drive Electronics (‘IDE’)adapters, Small Computer System Interface (‘SCSI’) adapters, and othersas will occur to those of skill in the art. Non-volatile computer memoryalso may be implemented for as an optical disk drive, electricallyerasable programmable read-only memory (so-called ‘EEPROM’ or ‘Flash’memory), RAM drives, and so on, as will occur to those of skill in theart.

Computer 10 also includes one or more input/output (‘I/O’) adapters 42,which implement user-oriented input/output through, for example,software drivers and computer hardware for controlling output to displaydevices such as computer display screens, as well as user input fromuser input devices 44 such as keyboards and mice. In addition, computer10 includes a communications adapter 46 for data communications withother computers 48 and for data communications with a datacommunications network 50. Such data communications may be carried outserially through RS-232 connections, through external buses such as aUniversal Serial Bus (‘USB’), through data communications datacommunications networks such as IP data communications networks, and inother ways as will occur to those of skill in the art. Communicationsadapters implement the hardware level of data communications throughwhich one computer sends data communications to another computer,directly or through a data communications network. Examples ofcommunications adapters suitable for use in computer 10 include modemsfor wired dial-up communications, Ethernet (IEEE 802.3) adapters forwired data communications network communications, and 802.11 adaptersfor wireless data communications network communications.

For further explanation, FIG. 2 sets forth a functional block diagram ofan example NOC 102 according to embodiments of the present invention.The NOC in FIG. 2 is implemented on a ‘chip’ 100, that is, on anintegrated circuit. NOC 102 includes integrated processor (‘IP’) blocks104, routers 110, memory communications controllers 106, and networkinterface controllers 108 grouped into interconnected nodes. Each IPblock 104 is adapted to a router 110 through a memory communicationscontroller 106 and a network interface controller 108. Each memorycommunications controller controls communications between an IP blockand memory, and each network interface controller 108 controls inter-IPblock communications through routers 110.

In NOC 102, each IP block represents a reusable unit of synchronous orasynchronous logic design used as a building block for data processingwithin the NOC. The term ‘IP block’ is sometimes expanded as‘intellectual property block,’ effectively designating an IP block as adesign that is owned by a party, that is the intellectual property of aparty, to be licensed to other users or designers of semiconductorcircuits. In the scope of the present invention, however, there is norequirement that IP blocks be subject to any particular ownership, sothe term is always expanded in this specification as ‘integratedprocessor block.’ IP blocks, as specified here, are reusable units oflogic, cell, or chip layout design that may or may not be the subject ofintellectual property. IP blocks are logic cores that can be formed asASIC chip designs or FPGA logic designs.

One way to describe IP blocks by analogy is that IP blocks are for NOCdesign what a library is for computer programming or a discreteintegrated circuit component is for printed circuit board design. InNOCs consistent with embodiments of the present invention, IP blocks maybe implemented as generic gate netlists, as complete special purpose orgeneral purpose microprocessors, or in other ways as may occur to thoseof skill in the art. A netlist is a Boolean-algebra representation(gates, standard cells) of an IP block's logical-function, analogous toan assembly-code listing for a high-level program application. NOCs alsomay be implemented, for example, in synthesizable form, described in ahardware description language such as Verilog or VHDL. In addition tonetlist and synthesizable implementation, NOCs also may be delivered inlower-level, physical descriptions. Analog IP block elements such asSERDES, PLL, DAC, ADC, and so on, may be distributed in atransistor-layout format such as GDSII. Digital elements of IP blocksare sometimes offered in layout format as well. It will also beappreciated that IP blocks, as well as other logic circuitry implementedconsistent with the invention may be distributed in the form of computerdata files, e.g., logic definition program code, that define at variouslevels of detail the functionality and/or layout of the circuitarrangements implementing such logic. Thus, while the invention has andhereinafter will be described in the context of circuit arrangementsimplemented in fully functioning integrated circuit devices, dataprocessing systems utilizing such devices, and other tangible, physicalhardware circuits, those of ordinary skill in the art having the benefitof the instant disclosure will appreciate that the invention may also beimplemented within a program product, and that the invention appliesequally regardless of the particular type of computer readable storagemedium being used to distribute the program product. Examples ofcomputer readable storage media include, but are not limited to,physical, recordable type media such as volatile and non-volatile memorydevices, floppy disks, hard disk drives, CD-ROMs, and DVDs (amongothers).

Each IP block 104 in the example of FIG. 2 is adapted to a router 110through a memory communications controller 106. Each memorycommunication controller is an aggregation of synchronous andasynchronous logic circuitry adapted to provide data communicationsbetween an IP block and memory. Examples of such communications betweenIP blocks and memory include memory load instructions and memory storeinstructions. The memory communications controllers 106 are described inmore detail below with reference to FIG. 3. Each IP block 104 is alsoadapted to a router 110 through a network interface controller 108,which controls communications through routers 110 between IP blocks 104.Examples of communications between IP blocks include messages carryingdata and instructions for processing the data among IP blocks inparallel applications and in pipelined applications. The networkinterface controllers 108 are also described in more detail below withreference to FIG. 3.

Routers 110, and the corresponding links 118 therebetween, implement thenetwork operations of the NOC. The links 118 may be packet structuresimplemented on physical, parallel wire buses connecting all the routers.That is, each link may be implemented on a wire bus wide enough toaccommodate simultaneously an entire data switching packet, includingall header information and payload data. If a packet structure includes64 bytes, for example, including an eight byte header and 56 bytes ofpayload data, then the wire bus subtending each link is 64 bytes wide,512 wires. In addition, each link may be bi-directional, so that if thelink packet structure includes 64 bytes, the wire bus actually contains1024 wires between each router and each of its neighbors in the network.In such an implementation, a message could include more than one packet,but each packet would fit precisely onto the width of the wire bus. Inthe alternative, a link may be implemented on a wire bus that is onlywide enough to accommodate a portion of a packet, such that a packetwould be broken up into multiple beats, e.g., so that if a link isimplemented as 16 bytes in width, or 128 wires, a 64 byte packet couldbe broken into four beats. It will be appreciated that differentimplementations may use different bus widths based on practical physicallimits as well as desired performance characteristics. If the connectionbetween the router and each section of wire bus is referred to as aport, then each router includes five ports, one for each of fourdirections of data transmission on the network and a fifth port foradapting the router to a particular IP block through a memorycommunications controller and a network interface controller.

Each memory communications controller 106 controls communicationsbetween an IP block and memory. Memory can include off-chip main RAM112, memory 114 connected directly to an IP block through a memorycommunications controller 106, on-chip memory enabled as an IP block116, and on-chip caches. In NOC 102, either of the on-chip memories 114,116, for example, may be implemented as on-chip cache memory. All theseforms of memory can be disposed in the same address space, physicaladdresses or virtual addresses, true even for the memory attacheddirectly to an IP block. Memory addressed messages therefore can beentirely bidirectional with respect to IP blocks, because such memorycan be addressed directly from any IP block anywhere on the network.Memory 116 on an IP block can be addressed from that IP block or fromany other IP block in the NOC. Memory 114 attached directly to a memorycommunication controller can be addressed by the IP block that isadapted to the network by that memory communication controller—and canalso be addressed from any other IP block anywhere in the NOC.

NOC 102 includes two memory management units (‘MMUs’) 120, 122,illustrating two alternative memory architectures for NOCs consistentwith embodiments of the present invention. MMU 120 is implemented withinan IP block, allowing a processor within the IP block to operate invirtual memory while allowing the entire remaining architecture of theNOC to operate in a physical memory address space. MMU 122 isimplemented off-chip, connected to the NOC through a data communicationsport 124. The port 124 includes the pins and other interconnectionsrequired to conduct signals between the NOC and the MMU, as well assufficient intelligence to convert message packets from the NOC packetformat to the bus format required by the external MMU 122. The externallocation of the MMU means that all processors in all IP blocks of theNOC can operate in virtual memory address space, with all conversions tophysical addresses of the off-chip memory handled by the off-chip MMU122.

In addition to the two memory architectures illustrated by use of theMMUs 120, 122, data communications port 126 illustrates a third memoryarchitecture useful in NOCs capable of being utilized in embodiments ofthe present invention. Port 126 provides a direct connection between anIP block 104 of the NOC 102 and off-chip memory 112. With no MMU in theprocessing path, this architecture provides utilization of a physicaladdress space by all the IP blocks of the NOC. In sharing the addressspace bi-directionally, all the IP blocks of the NOC can access memoryin the address space by memory-addressed messages, including loads andstores, directed through the IP block connected directly to the port126. The port 126 includes the pins and other interconnections requiredto conduct signals between the NOC and the off-chip memory 112, as wellas sufficient intelligence to convert message packets from the NOCpacket format to the bus format required by the off-chip memory 112.

In the example of FIG. 2, one of the IP blocks is designated a hostinterface processor 128. A host interface processor 128 provides aninterface between the NOC and a host computer 10 in which the NOC may beinstalled and also provides data processing services to the other IPblocks on the NOC, including, for example, receiving and dispatchingamong the IP blocks of the NOC data processing requests from the hostcomputer. A NOC may, for example, implement a video graphics adapter 26or a coprocessor 28 on a larger computer 10 as described above withreference to FIG. 1. In the example of FIG. 2, the host interfaceprocessor 128 is connected to the larger host computer through a datacommunications port 130. The port 130 includes the pins and otherinterconnections required to conduct signals between the NOC and thehost computer, as well as sufficient intelligence to convert messagepackets from the NOC to the bus format required by the host computer 10.In the example of the NOC coprocessor in the computer of FIG. 1, such aport would provide data communications format translation between thelink structure of the NOC coprocessor 28 and the protocol required forthe front side bus 36 between the NOC coprocessor 28 and the bus adapter18.

FIG. 3 next illustrates a functional block diagram illustrating ingreater detail the components implemented within an IP block 104, memorycommunications controller 106, network interface controller 108 androuter 110 in NOC 102, collectively illustrated at 132. IP block 104includes a computer processor 134 and I/O functionality 136. In thisexample, computer memory is represented by a segment of random accessmemory (‘RAM’) 138 in IP block 104. The memory, as described above withreference to FIG. 2, can occupy segments of a physical address spacewhose contents on each IP block are addressable and accessible from anyIP block in the NOC. The processors 134, I/O capabilities 136, andmemory 138 in each IP block effectively implement the IP blocks asgenerally programmable microcomputers. As explained above, however, inthe scope of the present invention, IP blocks generally representreusable units of synchronous or asynchronous logic used as buildingblocks for data processing within a NOC. Implementing IP blocks asgenerally programmable microcomputers, therefore, although a commonembodiment useful for purposes of explanation, is not a limitation ofthe present invention.

In NOC 102 of FIG. 3, each memory communications controller 106 includesa plurality of memory communications execution engines 140. Each memorycommunications execution engine 140 is enabled to execute memorycommunications instructions from an IP block 104, includingbidirectional memory communications instruction flow 141, 142, 144between the network and the IP block 104. The memory communicationsinstructions executed by the memory communications controller mayoriginate, not only from the IP block adapted to a router through aparticular memory communications controller, but also from any IP block104 anywhere in NOC 102. That is, any IP block in the NOC can generate amemory communications instruction and transmit that memorycommunications instruction through the routers of the NOC to anothermemory communications controller associated with another IP block forexecution of that memory communications instruction. Such memorycommunications instructions can include, for example, translationlookaside buffer control instructions, cache control instructions,barrier instructions, and memory load and store instructions.

Each memory communications execution engine 140 is enabled to execute acomplete memory communications instruction separately and in parallelwith other memory communications execution engines. The memorycommunications execution engines implement a scalable memory transactionprocessor optimized for concurrent throughput of memory communicationsinstructions. Memory communications controller 106 supports multiplememory communications execution engines 140 all of which runconcurrently for simultaneous execution of multiple memorycommunications instructions. A new memory communications instruction isallocated by the memory communications controller 106 to a memorycommunications engine 140 and memory communications execution engines140 can accept multiple response events simultaneously. In this example,all of the memory communications execution engines 140 are identical.Scaling the number of memory communications instructions that can behandled simultaneously by a memory communications controller 106,therefore, is implemented by scaling the number of memory communicationsexecution engines 140.

In NOC 102 of FIG. 3, each network interface controller 108 is enabledto convert communications instructions from command format to networkpacket format for transmission among the IP blocks 104 through routers110. The communications instructions may be formulated in command formatby the IP block 104 or by memory communications controller 106 andprovided to the network interface controller 108 in command format. Thecommand format may be a native format that conforms to architecturalregister files of IP block 104 and memory communications controller 106.The network packet format is typically the format required fortransmission through routers 110 of the network. Each such message iscomposed of one or more network packets. Examples of such communicationsinstructions that are converted from command format to packet format inthe network interface controller include memory load instructions andmemory store instructions between IP blocks and memory. Suchcommunications instructions may also include communications instructionsthat send messages among IP blocks carrying data and instructions forprocessing the data among IP blocks in parallel applications and inpipelined applications.

In NOC 102 of FIG. 3, each IP block is enabled to sendmemory-address-based communications to and from memory through the IPblock's memory communications controller and then also through itsnetwork interface controller to the network. A memory-address-basedcommunications is a memory access instruction, such as a loadinstruction or a store instruction, that is executed by a memorycommunication execution engine of a memory communications controller ofan IP block. Such memory-address-based communications typicallyoriginate in an IP block, formulated in command format, and handed offto a memory communications controller for execution.

Many memory-address-based communications are executed with messagetraffic, because any memory to be accessed may be located anywhere inthe physical memory address space, on-chip or off-chip, directlyattached to any memory communications controller in the NOC, orultimately accessed through any IP block of the NOC—regardless of whichIP block originated any particular memory-address-based communication.Thus, in NOC 102, all memory-address-based communications that areexecuted with message traffic are passed from the memory communicationscontroller to an associated network interface controller for conversionfrom command format to packet format and transmission through thenetwork in a message. In converting to packet format, the networkinterface controller also identifies a network address for the packet independence upon the memory address or addresses to be accessed by amemory-address-based communication. Memory address based messages areaddressed with memory addresses. Each memory address is mapped by thenetwork interface controllers to a network address, typically thenetwork location of a memory communications controller responsible forsome range of physical memory addresses. The network location of amemory communication controller 106 is naturally also the networklocation of that memory communication controller's associated router110, network interface controller 108, and IP block 104. The instructionconversion logic 150 within each network interface controller is capableof converting memory addresses to network addresses for purposes oftransmitting memory-address-based communications through routers of aNOC.

Upon receiving message traffic from routers 110 of the network, eachnetwork interface controller 108 inspects each packet for memoryinstructions. Each packet containing a memory instruction is handed tothe memory communications controller 106 associated with the receivingnetwork interface controller, which executes the memory instructionbefore sending the remaining payload of the packet to the IP block forfurther processing. In this way, memory contents are always prepared tosupport data processing by an IP block before the IP block beginsexecution of instructions from a message that depend upon particularmemory content.

In NOC 102 of FIG. 3, each IP block 104 is enabled to bypass its memorycommunications controller 106 and send inter-IP block, network-addressedcommunications 146 directly to the network through the IP block'snetwork interface controller 108. Network-addressed communications aremessages directed by a network address to another IP block. Suchmessages transmit working data in pipelined applications, multiple datafor single program processing among IP blocks in a SIMD application, andso on, as will occur to those of skill in the art. Such messages aredistinct from memory-address-based communications in that they arenetwork addressed from the start, by the originating IP block whichknows the network address to which the message is to be directed throughrouters of the NOC. Such network-addressed communications are passed bythe IP block through I/O functions 136 directly to the IP block'snetwork interface controller in command format, then converted to packetformat by the network interface controller and transmitted throughrouters of the NOC to another IP block. Such network-addressedcommunications 146 are bi-directional, potentially proceeding to andfrom each IP block of the NOC, depending on their use in any particularapplication. Each network interface controller, however, is enabled toboth send and receive such communications to and from an associatedrouter, and each network interface controller is enabled to both sendand receive such communications directly to and from an associated IPblock, bypassing an associated memory communications controller 106.

Each network interface controller 108 in the example of FIG. 3 is alsoenabled to implement virtual channels on the network, characterizingnetwork packets by type. Each network interface controller 108 includesvirtual channel implementation logic 148 that classifies eachcommunication instruction by type and records the type of instruction ina field of the network packet format before handing off the instructionin packet form to a router 110 for transmission on the NOC. Examples ofcommunication instruction types include inter-IP blocknetwork-address-based messages, request messages, responses to requestmessages, invalidate messages directed to caches; memory load and storemessages; and responses to memory load messages, etc.

Each router 110 in the example of FIG. 3 includes routing logic 152,virtual channel control logic 154, and virtual channel buffers 156. Therouting logic typically is implemented as a network of synchronous andasynchronous logic that implements a data communications protocol stackfor data communication in the network formed by the routers 110, links118, and bus wires among the routers. Routing logic 152 includes thefunctionality that readers of skill in the art might associate inoff-chip networks with routing tables, routing tables in at least someembodiments being considered too slow and cumbersome for use in a NOC.Routing logic implemented as a network of synchronous and asynchronouslogic can be configured to make routing decisions as fast as a singleclock cycle. The routing logic in this example routes packets byselecting a port for forwarding each packet received in a router. Eachpacket contains a network address to which the packet is to be routed.

In describing memory-address-based communications above, each memoryaddress was described as mapped by network interface controllers to anetwork address, a network location of a memory communicationscontroller. The network location of a memory communication controller106 is naturally also the network location of that memory communicationcontroller's associated router 110, network interface controller 108,and IP block 104. In inter-IP block, or network-address-basedcommunications, therefore, it is also typical for application-level dataprocessing to view network addresses as the location of an IP blockwithin the network formed by the routers, links, and bus wires of theNOC. FIG. 2 illustrates that one organization of such a network is amesh of rows and columns in which each network address can beimplemented, for example, as either a unique identifier for each set ofassociated router, IP block, memory communications controller, andnetwork interface controller of the mesh or x, y coordinates of eachsuch set in the mesh.

In NOC 102 of FIG. 3, each router 110 implements two or more virtualcommunications channels, where each virtual communications channel ischaracterized by a communication type. Communication instruction types,and therefore virtual channel types, include those mentioned above:inter-IP block network-address-based messages, request messages,responses to request messages, invalidate messages directed to caches;memory load and store messages; and responses to memory load messages,and so on. In support of virtual channels, each router 110 in theexample of FIG. 3 also includes virtual channel control logic 154 andvirtual channel buffers 156. The virtual channel control logic 154examines each received packet for its assigned communications type andplaces each packet in an outgoing virtual channel buffer for thatcommunications type for transmission through a port to a neighboringrouter on the NOC.

Each virtual channel buffer 156 has finite storage space. When manypackets are received in a short period of time, a virtual channel buffercan fill up—so that no more packets can be put in the buffer. In otherprotocols, packets arriving on a virtual channel whose buffer is fullwould be dropped. Each virtual channel buffer 156 in this example,however, is enabled with control signals of the bus wires to advisesurrounding routers through the virtual channel control logic to suspendtransmission in a virtual channel, that is, suspend transmission ofpackets of a particular communications type. When one virtual channel isso suspended, all other virtual channels are unaffected—and can continueto operate at full capacity. The control signals are wired all the wayback through each router to each router's associated network interfacecontroller 108. Each network interface controller is configured to, uponreceipt of such a signal, refuse to accept, from its associated memorycommunications controller 106 or from its associated IP block 104,communications instructions for the suspended virtual channel. In thisway, suspension of a virtual channel affects all the hardware thatimplements the virtual channel, all the way back up to the originatingIP blocks.

One effect of suspending packet transmissions in a virtual channel isthat no packets are ever dropped. When a router encounters a situationin which a packet might be dropped in some unreliable protocol such as,for example, the Internet Protocol, the routers in the example of FIG. 3may suspend by their virtual channel buffers 156 and their virtualchannel control logic 154 all transmissions of packets in a virtualchannel until buffer space is again available, eliminating any need todrop packets. The NOC of FIG. 3, therefore, may implement highlyreliable network communications protocols with an extremely thin layerof hardware.

The example NOC of FIG. 3 may also be configured to maintain cachecoherency between both on-chip and off-chip memory caches. Each NOC cansupport multiple caches each of which operates against the sameunderlying memory address space. For example, caches may be controlledby IP blocks, by memory communications controllers, or by cachecontrollers external to the NOC. Either of the on-chip memories 114, 116in the example of FIG. 2 may also be implemented as an on-chip cache,and, within the scope of the present invention, cache memory can beimplemented off-chip also.

Each router 110 illustrated in FIG. 3 includes five ports, four ports158A-D connected through bus wires 118 to other routers and a fifth port160 connecting each router to its associated IP block 104 through anetwork interface controller 108 and a memory communications controller106. As can be seen from the illustrations in FIGS. 2 and 3, the routers110 and the links 118 of the NOC 102 form a mesh network with verticaland horizontal links connecting vertical and horizontal ports in eachrouter. In the illustration of FIG. 3, for example, ports 158A, 158C and160 are termed vertical ports, and ports 158B and 158D are termedhorizontal ports.

FIG. 4 next illustrates in another manner one exemplary implementationof an IP block 104 consistent with the invention, implemented as aprocessing element partitioned into an issue or instruction unit (IU)162, execution unit (XU) 164 and auxiliary execution unit (AXU) 166. Inthe illustrated implementation, IU 162 includes a plurality ofinstruction buffers 168 that receive instructions from an L1 instructioncache (iCACHE) 170. Each instruction buffer 168 is dedicated to one of aplurality, e.g., four, symmetric multithreaded (SMT) hardware threads.An effective-to-real translation unit (iERAT) 172 is coupled to iCACHE170, and is used to translate instruction fetch requests from aplurality of thread fetch sequencers 174 into real addresses forretrieval of instructions from lower order memory. Each thread fetchsequencer 174 is dedicated to a particular hardware thread, and is usedto ensure that instructions to be executed by the associated thread isfetched into the iCACHE for dispatch to the appropriate execution unit.As also shown in FIG. 4, instructions fetched into instruction buffer168 may also be monitored by branch prediction logic 176, which provideshints to each thread fetch sequencer 174 to minimize instruction cachemisses resulting from branches in executing threads.

IU 162 also includes a dependency/issue logic block 178 dedicated toeach hardware thread, and configured to resolve dependencies and controlthe issue of instructions from instruction buffer 168 to XU 164. Inaddition, in the illustrated embodiment, separate dependency/issue logic180 is provided in AXU 166, thus enabling separate instructions to beconcurrently issued by different threads to XU 164 and AXU 166. In analternative embodiment, logic 180 may be disposed in IU 162, or may beomitted in its entirety, such that logic 178 issues instructions to AXU166.

XU 164 is implemented as a fixed point execution unit, including a setof general purpose registers (GPR's) 182 coupled to fixed point logic184, branch logic 186 and load/store logic 188. Load/store logic 188 iscoupled to an L1 data cache (dCACHE) 190, with effective to realtranslation provided by dERAT logic 192. XU 164 may be configured toimplement practically any instruction set, e.g., all or a portion of a32b or 64b PowerPC instruction set.

AXU 166 operates as an auxiliary execution unit including dedicateddependency/issue logic 180 along with one or more execution blocks 194.AXU 166 may include any number of execution blocks, and may implementpractically any type of execution unit, e.g., a floating point unit, orone or more specialized execution units such as encryption/decryptionunits, coprocessors, vector processing units, graphics processing units,XML processing units, etc. In the illustrated embodiment, AXU 166includes a high speed auxiliary interface to XU 164, e.g., to supportdirect moves between AXU architected state and XU architected state.

Communication with IP block 104 may be managed in the manner discussedabove in connection with FIG. 2, via network interface controller 108coupled to NOC 102. Address-based communication, e.g., to access L2cache memory, may be provided, along with message-based communication.For example, each IP block 104 may include a dedicated in box and/or outbox in order to handle inter-node communications between IP blocks.

Embodiments of the present invention may be implemented within thehardware and software environment described above in connection withFIGS. 1-4. However, it will be appreciated by one of ordinary skill inthe art having the benefit of the instant disclosure that the inventionmay be implemented in a multitude of different environments, and thatother modifications may be made to the aforementioned hardware andsoftware embodiment without departing from the spirit and scope of theinvention. As such, the invention is not limited to the particularhardware and software environment disclosed herein.

Local Instruction Loop Buffer Utilizing Execution Unit Register File

Power consumption in modern System on Chip (SOC) designs has become asignificant design constraint as more complex chips are developed andtechnologies shrink to allow for more logic per chip. The threshold hasnow been passed where it costs more power per bit to move the bit frommemory to the CPU, than it does to perform the desired computation.Therefore, new power reduction mechanisms are needed for moving dataaround on, and off, chip. Additionally, many features once unique todigital signal processors (DSPs) are increasingly being implemented ongeneral purpose processing units to reduce cost by eliminating the needfor separate DSP chips in a system and to increase performance byeliminating the need to move data between the DSP chip and the CPU. ManyDSP algorithms typically performed by DSP chips such as Fast FourierTransforms (FFT) do not perform as well with traditional general purposeprocessing units, and although some features added to general purposeprocessing units such as single instruction multiple data (SIMD)execution units and predication have improved performance, the powerconsumption of processing units performing these algorithms is stillmuch higher than that of DSP chips specifically tailored for thosealgorithms, often because general purpose processing units typicallycontain large blocks of logic such as the L1 data cache that areintended to improve performance generally for most workloads. However,for many DSP algorithms, this logic does very little to improveperformance.

For example, many DSP algorithms follow a similar coding pattern, suchas represented in Table I below:

TABLE I DSP Algorithm Pseudocode loop:    addi counter, 1 # increment acounter    load v0    load v1    load v2    load v3    load v4    ...(load as much data as possible into regfile)    vmath    vmath    vmath   vmath    ... (perform the algorithm)    store v0    store v1    storev2    store v3    store v4    ... (store the processed data out tomemory)    be start counter # (loop)

DSP algorithms similar to the above typically benefit from largeregister files, to be able to fit as much data as possible locally.Additionally, the algorithms typically do not really benefit from an L1data cache as all of the data is typically new data that is not reusedafter it is written back into the register file. When such an algorithmis performed in a general purpose processing unit, however, the data isstill brought through the L1 data cache, which adds significantly to thepower consumption of the processing unit when performing the algorithm.

Embodiments consistent with the invention, on the other hand, may add arelatively small amount of control logic to an existing general purposeprocessing unit design to implement a local instruction loop bufferwithin the register file of an execution unit to accelerate theperformance of DSP and other specialized algorithms with reducedoverhead in circuit logic outside of the execution unit, therebyenabling one or more functional units used in general purpose/non-DSPworkloads but that are otherwise unnecessary for DSP workloads to beeffectively disabled to reduce power consumption of the processing unitand/or to free such functional units to perform other workloads.

In the embodiments discussed hereinafter, for example, an AXU such as afloating point execution unit may be supplemented with several AXUinstructions, a few special purpose registers, and the capability tofetch, decode, and execute instructions directly from the AXU registerfile rather than the existing instruction cache. This allows for otherfunctional units in a processing unit to be powered down, of if neededfor other workloads, allows for greater overall performance as thefunctional units are not utilized by the AXU instructions.

FIG. 5, for example, illustrates an example general purpose processingunit 200, which may be implemented, for example, as an IP block from thecomputer of FIGS. 1-4. General purpose processing unit 200 includesinstruction logic 202 configured to supply instructions to one or moreexecution units, e.g., an auxiliary execution unit (AXU) 204(implemented, for example, as a floating point execution unit), firstand second fixed point execution units (XU0, XU1) 206, 208, and a loadstore unit (LSU) 210. An L1 cache is implemented as separate instruction(ICache) and data (DCache) caches 212, 214, each with a correspondingeffective-to-real address translation (ERAT) unit 216, 218. A memorymanagement unit (MMU) 220 controls the interface between the L1 and L2caches.

Issue logic 202 in the illustrated embodiment is multithreaded to enablethe concurrent execution of multiple instruction streams associated withmultiple threads of execution. In addition, issue logic 202 may includeseparate logic, e.g., logic 222, 224, for fetching, decoding and issuinginstructions for different execution units, and as such includes oneinstance 222 of issue logic components used to fetch, decode and issueinstructions to the fixed point execution units and load/store unit206-210, as well as a separate instance used to fetch, decode and issueinstructions to AXU 204. Each instance 222, 224 fetches instructionsfrom a shared instruction buffer 226, and processes the fetchedinstructions using dedicated decode logic 228 and rename logic 230.Branch prediction logic 232 is used to maintain historical informationfor a plurality of branch instructions and predict a code path to befollowed subsequent to execution of such branch instructions.

In addition, in order to implement a local instruction loop buffer,additional control logic 234 is provided, and optionally having thecapability, for example, to control the power to one or more voltageislands 236, e.g., to selectively disable and power down one or morefunctional units in general purpose processing unit 200. Additionalfunctionality for control logic 234 will become more apparent from thediscussion below.

It will be appreciated that a wide variety of alternate configurationsmay be utilized to that illustrated in FIG. 5. For example, variousalternative issue logic configurations may be used, e.g., where variouscomponents or stages are combined for different execution units orthreads of execution, rather than being implemented separately, or whereadditional and/or alternative components or stages are used. Differentmemory architectures and cache structures may be used in otherembodiments, as may different numbers and combinations of executionunits. Alternative control logic may be used to implement various typesof power reduction methodologies (e.g., to clock gate certain logicrather than powering such logic down). In addition, processing unit 200may be disposed within an IP block or a processor core that isintegrated along with other IP blocks/processor cores on the sameintegrated circuit device, or alternatively, may be the only processingunit integrated on an integrated circuit device.

In general, embodiments consistent with the invention may be utilized inpractically any general purpose processing unit configuration includingan execution unit with a register file. Therefore, the invention is notlimited to the particular implementations disclosed herein.

To implement a local instruction loop buffer, control logic 234 may beresponsive to a dedicated instruction in an instruction stream beingprocessed by processing unit 200 (e.g., a branch into AXU instruction,discussed in greater detail below), or alternatively, in response to aconfiguration or mode bit in a special purpose register in processingunit 200, which may be controlled via a write to the special purposeregister.

In this embodiment, control logic 234 may disable a number of functionalunits that are unnecessary for executing a DSP or other specializedalgorithm in AXU 204, e.g., one or both of execution units 206, 208, L1DCache 214 and DERAT 218. In addition, in some embodiments,multithreading may be disabled. In other embodiments, however, nofunctional units may be disabled when executing an algorithm in AXU 204,and as such, other workloads that utilize such functional units mayproceed in parallel with the algorithm executed in AXU 204.

Next, FIG. 6 illustrates one example implementation of control logic 234for implementing a local instruction loop buffer in AXU 204. Inparticular, in this embodiment control logic 234 includes an AXU loopbuffer controller 250 and selection logic 252, which may be implemented,for example, as a two input multiplexer.

Decode logic 228 of instance 224 is illustrated as including instructiondecode logic 254 and dependency logic 256, and selection logic 252 isinterposed between instruction buffer 226 and instruction decode logic254. Selection logic includes a first input that receives instructionsfrom instruction buffer 226 and a second input that receivesinstructions from an AXU register file 258 in AXU 204. Register file 258also provides operand data stored in one or more registers to an AXUexecution pipeline 260, and receives results of AXU operations back fromAXU execution pipeline 260. Instructions received either from aninstruction stream stored in instruction buffer 226 or from registerfile 258 are decoded by instruction decode logic 254, dependencies aretracked in dependency logic 256, and register renaming, if required, isperformed by register rename logic 230 (logic 230 may be omitted in someembodiments). Instructions are then issued by issue unit 262 toexecution pipeline 260, as well as to controller 250. In some instances,the dependency and rename logic may be more closely coupled than asshown in FIG. 6.

In some embodiments, AXU 204 is multithreaded, and as such, multiplethread-specific instances 228 may be provided, with issue unit 262arbitrating between the multiple threads/instances. In otherembodiments, however, AXU 204 may be single threaded.

Control logic 234 effectively implements a local instruction loop bufferin register file 258. In doing so, a portion of the registers/entries inregister file 258 are used to store instructions to be fetched, decodedand executed by AXU 204, while another portion of the registers/entriesin register file 258 are used to store data, e.g., operand data used asoperands to instructions executed by AXU 204 and/or result datagenerated from the execution of such instructions.

To implement a local instruction loop buffer, one or more additionalinstructions may be added to the instruction set for the AXU, andsupported by instruction decode logic 254. One such instruction isreferred to herein as a “branch into AXU” instruction, which may, forexample, optionally specify a particular register and/or word inregister file 258 representing the start of a sequence of instructionsstored in the register file. Another such instruction is referred toherein as a “branch out of AXU” instruction, which may specify a targetaddress in a number of manners known in the art, e.g., via a linkregister, an offset, indirect addressing, etc.

Selection logic 252, as noted above, feeds AXU instruction decoder logic254, and either takes as input instructions from AXU register file 258,or the existing instruction buffer 226/instruction cache 212 (FIG. 5). Amode signal, output by controller 250, controls selection logic 252, andmay be accessed and controlled by a software-accessible special purposeregister (SPR). The mode signal switches between an AXU register fetchmode, in which instructions are fetched from the local instruction loopbuffer, and a regular mode, where instructions are fetched frominstruction buffer 226. A “branch into AXU” instruction may activate theAXU register fetch mode for the thread, while a “branch out of AXUregister” may put it back in the regular mode.

Controller 250 is configured to sequentially fetch new instructions fromAXU register file 258 and manage fetches for AXU branches (i.e., takenv. non-taken). In this regard, control logic 234 may include a softwareaccessible AXU loop counter and a loop address SPR for storing the AXUregister file address (e.g., entry and word) of the head of the loop(i.e., the first instruction in a sequence of instructions).Alternatively, these registers may be written to in connection withexecution of a branch into AXU instruction, with the loop count and/orAXU register file address provided as operands to the instruction.

The loop counter is decremented each iteration of the sequence ofinstructions, and whenever the loop counter is non-zero, a branch istaken back to the address stored in the loop address SPR. When the loopcounter reaches zero, controller 250 may return the AXU to regular mode,thereby returning the AXU to fetching and executing instructions frominstruction buffer 226.

It will be appreciated that various alternative hardware configurationsmay be used to implement a local instruction loop buffer consistent withthe invention. For example, instructions stored in register file 258 maybe decoded using separate decode logic from that used to decode theinstructions stored in instruction buffer 226, and in some instances,the instructions stored in register file 258 may be formatteddifferently from other instructions in the instruction set used by theAXU, e.g., to conserve space in the register file, or to encodedifferent, application-specific instructions.

In addition, it will be appreciated that practically any type ofalgorithm may be implemented in a local instruction loop bufferconsistent with the invention. Moreover, depending upon how complexcontroller 250 is implemented, instruction sequences of varyingcomplexity may be used in different embodiments. While in theillustrated embodiment only linear sequences of instructions aresupported by controller 250, in other embodiments controller 250 mayalso include branch control logic to enable conditional branches to beprovided in an instruction sequence and used to provide multiple codepaths through the instruction sequence. It will also be appreciated thatimplementation of any of the aforementioned functionality withincontroller 250 would be well within the abilities of one of ordinaryskill in the art having the benefit of the instant disclosure.

Now turning to FIG. 7, an example DSP algorithm routine 270, used toperform a DSP or other specialized algorithm with the use of a localinstruction loop buffer, is further illustrated. The left column ofroutine 270, in particular, represents the execution of an instructionstream, e.g., as may be provided in a software program and stored in amemory, cached in instruction cache 212 and executed by processing unit200. The instruction stream in part sets up a local instruction loopbuffer in AXU register file 258, stores an instruction sequence in thebuffer/register file, loads operand data into AXU register file 258,triggers execution of the sequence of instructions stored in thebuffer/register file by AXU 204, and writes out results to memory. Theright column, in turn, represents the operations performed by AXU 204 inAXU fetch mode, e.g., as coordinated by controller 250.

Routine 270 begins in block 272 by loading a sequence of AXUinstructions into the AXU register file, e.g., using conventional loadinstructions targeting the AXU register file. The instruction sequencemay represent, for example, a specialized algorithm such as a DSPalgorithm. For example, the instructions may be configured to performall or a portion of a Fast Fourier Transform (FFT) calculation.

Next, in block 274, the AXU register file address, representing thestart or head of the instruction sequence in the AXU register file, isstored in the loop address SPR, e.g., using a conventional writeinstruction to the loop address SPR.

Next, it is assumed for the purposes of this example that the amount ofdata to be processed by the instruction sequence is greater than can bestored in the AXU register file at the same time, such that the datamust be processed in multiple data groups. Block 276 initiates a FORloop to process each data group, and for each such data group block 278loads the data group into the AXU register file, e.g., usingconventional load instructions targeting the AXU register file. Block280 then sets the number of iterations to perform, e.g., using aconventional write instruction to the AXU loop counter. Alternatively,this count could be set once, with the loop counter automaticallyrestored to the initial count whenever the loop counter reaches zero. Inaddition, as noted above, the loop counter and/or the AXU register fileaddress may be provided as operands to a branch into AXU instruction,eliminating the need for separate instructions to set either value.

Next, a branch into AXU instruction is executed in block 282. Thisinstruction is decoded by instruction decode logic 254, which transferscontrol to controller 250 to enable the AXU register fetch mode for AXU204 (block 284). Doing so causes controller 250 to control selectionlogic 252 to pass instructions from the instruction sequence stored inAXU register file 258 to instruction decode logic 254, resulting inexecution of the instruction sequence stored in the AXU register file byAXU execution pipeline 260 in block 286. The AXU loop counter is thendecremented in block 288, and if the counter is not zero, block 290returns control to block 286 to perform another iteration of theinstruction sequence. Once the loop counter reaches zero, however, block290 passes control to block 292 to disable the AXU register fetch modeand return selection logic 252 to pass instructions from instructionbuffer 226 to instruction decode logic 254.

The next instruction in the instruction stream is a branch out of AXUinstruction (block 294), followed by block 296, in which one or moreinstructions write out the results of the calculations performed by theinstruction sequence from the AXU register file to memory, or otherwisemanipulate the result data (e.g., where each instruction sequenceiteration calculates one or more partial sums, block 296 may simply sumthe result data with a running sum). Control then returns to block 276to process any additional data groups. Once all data groups have beenprocessed, routine 270 is complete.

As illustrated in FIG. 7, the branch out of AXU instruction may beincluded in the instruction stream. Alternatively, the branch out of AXUinstruction may be included in the instruction sequence and fetched fromthe AXU register file. As another alternative, no separate branch out ofAXU instruction may be used, whereby clearing the AXU register fetchmode in block 292 may operate as an implied branch out of AXUinstruction that returns control to the instruction stream to executethe next instruction after the branch into AXU instruction.

It will be appreciated that the instruction sequences stored in AXUregister file and executed by the AXU will be application specific, andmay include practically any sequence of instructions suitable forimplementing a desired algorithm. Therefore, the invention is notlimited to the particular implementation disclosed herein andillustrated in FIG. 7. In addition, while not shown in FIG. 7, it willbe appreciated that while AXU 204 is in the AXU register fetch mode,controller 250 may disable one or more functional units in processingunit 200 to reduce power consumption of the processing unit.

The embodiments illustrated herein therefore allow for greaterperformance for a general purpose processing unit handling DSP and otherspecialized algorithms, and in many instances reducing power consumptionwhen handling such algorithms, and while also not sacrificingperformance for other, general purpose workloads, or requiring the needof a specialized execution unit.

Various additional modifications may be made to the disclosedembodiments without departing from the spirit and scope of theinvention. Therefore, the invention lies in the claims hereinafterappended.

What is claimed is:
 1. A method of executing instructions in aprocessing unit of the type including an execution unit including aregister file and instruction decode logic including an instructionbuffer, the method comprising: fetching a first plurality ofinstructions from an instruction stream and stored in the instructionbuffer with the instruction decode logic of the processing unit; inresponse to at least one instruction from the instruction stream,causing the execution unit to manipulate operand data stored in theregister file; in response to at least one instruction from theinstruction stream, loading a second plurality of instructions into theregister file of the execution unit to utilize at least a portion of theregister file as a local instruction loop buffer; fetching the secondplurality of instructions from the register file rather than fetchinginstructions stored in the instruction buffer; and causing the executionunit to execute the second plurality of instructions fetched from theregister file.
 2. The method of claim 1, further comprising: in responseto at least one instruction from the instruction stream, storing a loopcount value in a loop counter; and causing the execution unit to executethe second plurality of instructions for a plurality of iterations basedupon the loop count value stored in the loop counter.
 3. The method ofclaim 2, further comprising: in response to at least one instructionfrom the instruction stream, storing a loop address in a loop addressregister that points to a first instruction among the second pluralityof instructions in the register file; and branching to the firstinstruction at the end of the second plurality of instructions basedupon the loop address register.
 4. The method of claim 1, wherein theprocessing unit further comprises selection logic configured to passinstructions to the instruction decode logic, the selection logicincluding at least first and second inputs, the first input configuredto receive instructions from an instruction cache and the second inputconfigured to receive instructions from the register file, and whereinthe method further comprises switching the selection logic to the secondinput prior to fetching the second plurality of instructions.
 5. Themethod of claim 4, wherein the issue logic is configured to issueinstructions associated with a plurality of threads to the executionunit, wherein the instruction decode logic is associated with a firstthread among the plurality of threads, and wherein fetching the secondplurality of instructions is performed concurrently with fetchinginstructions from a second instruction stream associated with a secondthread among the plurality of threads.
 6. The method of claim 4, whereinthe instruction decode logic is configured to decode instructions froman instruction set associated with the execution unit, wherein theinstruction set includes a first instruction configured to branch intothe local instruction loop buffer and a second instruction configured tobranch out of the local instruction loop buffer.
 7. The method of claim1, wherein the execution unit is selected from the group consisting ofan auxiliary execution unit, a floating point execution unit, and afixed point execution unit.
 8. The method of claim 1, wherein theinstructions stored in the register file implement a digital signalprocessing (DSP) algorithm.
 9. The method of claim 1, further comprisingdisabling at least one functional unit in the processing unit when theexecution unit is executing the second plurality of instructions toreduce power consumption of the processing unit.